Manual processing to determine the cellular/biological content of various types of samples, and in particular samples that contain living cells, is cost-prohibitive in many applications and is also prone to errors. Automation is also cost-prohibitive in many applications, and is inappropriate as currently practiced—using, for example, liquid handling robots—for applications such as point-of-care or doctor's office analysis. As a result, there is an unmet need to provide sample processing for multiplexed biological assays that is less expensive and less prone to error than current automation or manual processing.
Certain Point-of-Care diagnostic assay systems based on centrifugal microfluidic technology are quite good at performing the necessary integrated sample preparation and assay measurement steps. This centrifugal microfluidic platform with optical detection allows for a variety of assay technologies to be implemented in parallel using a single instrument and disposable suite.
Gating or valving of liquids is a key feature of most centrifugal fluidic platforms, with a variety of different such means existing. These include but not limited to the use of siphoning; passive single-use valves based on surface tension effects (capillary valves, hydrophobic valves); single-use valves based on solid-to-liquid phase transition or melting of a “plug” due to heat applied by a contact heater or light source; and multiple-use valves based on the same principals. Some of these valving mechanisms are well known in the art and have entered the public domain, as described in U.S. Pat. No. 5,693,233, Abaxis.
It is recognized in the art, such as U.S. Pat. No. 6,143,248, Camera, that manipulation of liquid properties (surface tension, density), material properties (contact angle); and geometric parameters such as the capillary dimensions and configuration of the fluids on the disc; results in well-defined rotational velocities at which capillary pressure is ‘defeated’ and liquid ‘bursts’ through passive valves. Using these relationships, a wide range of relevant biological fluids and reagents may be gated at rotational rates from a few hundred RPM (revolutions per minute) to more than 5000 RPM. Siphons function in this way: A chamber is provided by an outlet channel which proceeds radially inward from the chamber and whose path doubles backwards, forming a U, and thus points radially outward. The “U” of the channel is at a radius inward of where the liquid meniscus of the defined volume of liquid which is to be resident in the chamber when the disc is under rotation. In this way, liquid does not proceed through the siphon at high rotational velocities.
As rotational velocity is decreased, capillary action may be used to imbibe the liquid within the siphon. The liquid is drawn past the U, until the liquid meniscus is at a point radially-outward of the position of the radially-inward meniscus of liquid filling the chamber. Upon increased rotational velocity, the meniscus in the channel acts to “pull” liquid from the chamber, just as liquid in a siphon under the influence of gravity may be used to empty a container by first rising above the container surface, and then dropping below it. The container will be emptied completely if the outlet of the channel is below the lowermost portion of the container. In the same way, the siphon on a centrifugal disc can be seen to function.
Siphons are useful because they act in an opposite fashion from capillary valves: Fluid will not flow past a siphon at a high rotational speed, unless the velocity is first decreased to allow capillary action to occur. This is especially useful for high velocity separation processes such as separation from plasma from whole blood, where the separation time is minimized if the rotational velocity is maximized. As a result, it is desirable to have valves at various points in the process which will not allow flow, no matter how great the rotational speed.
US 2004/0209374, Abaxis, requires that the location of the inner radial bend of a siphon is located closer to the centre than the holding structure that feeds it. This design relies on a smooth and hydrophilic surface to enable capillary action defeat the valve. US 2011/0094600, Bergeron et al, refer to standard siphons in the art and introduces a serial siphon valve design.
PCT Patent publication number WO02/074438, assigned to Gyros AB, discloses a microfluidic device that comprises several microchannel structures. A paper published by Gorkin et al ‘Centrifugal microfluidics for biomedical applications’ Lab on a Chip, Royal Society of Chemistry, col. 10, 28 May 2010, Pages 1758-1773 discloses a centrifugal microfluidic platform for biomedical applications having a siphon having a hydrophilic surface. A problem with siphons described in the art is that liquids cannot be retained within the chamber at low rotational velocities. For example, if the chamber is to be used for an incubation step where slow agitations are required, the meniscus may be drawn by capillary action around the “U” and the chamber emptied upon acceleration of the disc. Similarly, if a detection step is required where the disc must be stationary, the siphon will be defeated.
A further problem is the necessity for capillary action. This typically requires that the surface of the channel be smooth and hydrophilic. The latter is usually accomplished by surface treatment, e.g., plasma etching or deposition of hydrophilic materials. This adds to the costs and complexities of disc manufacture.
It is therefore an object of this invention to provide a valving system for use on a centrifugal microfluidic platform to overcome at least one of the above mentioned problems.